1. Field of the Invention
The invention relates generally to position or proximity sensors. More particularly, the invention is directed to an apparatus and method for detecting the position of a semiconductor wafer (hereinafter xe2x80x9cwaferxe2x80x9d) within a semiconductor fabrication system.
2. Description of Related Art
Nowadays, semiconductor fabrication systems are almost entirely automated. Typically, an unprocessed semiconductor wafer is processed into semiconductor micro-chips by automatically exposing each wafer to a number of distinct processes, such as photo masking, etching, or the like. Each wafer is a thin slice of base material, typically silicon, cut from an ingot or xe2x80x9cboule.xe2x80x9d The most common wafer diameters are 200 mm or 300 mm (xc2x11 mm), having a thickness of approximately 0.85 mm. Because of the nature of the base material and the thinness of each slice, the wafers can easily be damaged through mishandling.
Modem semiconductor fabrication systems typically utilize cluster tools having multiple process chambers clustered around a central chamber housing a wafer handling robot. Various semiconductor fabrication processes occur at each process chamber, such as degassing, substrate pre-conditioning, cool down, chemical vapor deposition, physical vapor deposition, etching, or the like. These cluster tools also typically includes one or more cassettes in which multiple wafers are stacked prior to and after fabrication. The cassette is typically passed into the cluster tool through a loading chamber, or load-lock. The centrally located wafer handling robot has access to the multiple process chambers and the loading chamber through ports coupling each chamber to the central chamber.
Furthermore, the cluster tool forms a sealed environment that is controlled to limit potential contamination of the wafers and to ensure that optimal processing conditions are maintained. Examples of cluster tools can be found in U.S. Pat. Nos. 5,955,858, 5,447,409, and 5,469,035, all of which are incorporated herein by reference.
To increase fabrication efficiency, a high throughput of wafers through each cluster tool is desirable. One of the ways to achieve a high throughput is by increasing the speed that each wafer is transported between process chambers by the wafer handling robot, i.e., reducing the time between individual processes. An increased handling speed, however, escalates the potential for a wafer to dislodge from a clamping mechanism holding the wafer at the distal end of the wafer handling robot. If a wafer were to dislodge, not only will the wafer be damaged, but it may damage the entire cluster tool and negatively impact the overall throughput. It is therefore desirable to accurately sense the position of the wafer handling robot and more importantly the position of a wafer carried by the wafer handling robot. Furthermore, true automation of the semiconductor fabrication process requires knowing the location of the wafer handling robot and/or the wafer at all times.
Detecting the position of the wafer handling robot and/or wafer is subject to a number of criteria, such as:
it must be determined whether each wafer is securely grasped or clamped by the wafer handling robot, but not overly so, so as to damage the fragile wafer;
it must be determined that the clamping and placement of each wafer is precise and accurate since any misplacement might negatively impact a process and/or damage the wafer;
the position or proximity sensor must be heat resistant, as some of the processes may expose it to high temperatures;
the position or proximity sensor must not introduce any particulates or contaminants into the closed environment that can ultimately damage the wafer or semiconductors (it has been found that particulates as small as the critical dimension or line width of a semiconductor device, can damage the integrity of an integrated circuit formed on a wafer); and
it must be determined if a wafer is dislodged from the clamping mechanism so that the wafer handling robot can be halted before further damage can occur.
Currently, the position of a wafer is sensed by optical detection devices. Several such optical detection devices are disclosed in U.S. Pat. Nos. 5,563,798; 5,740,062; and 5,796,486. A generic optical detection device 100 is shown in FIG. 1. The primary components of such an optical detection device include a light emitting element 102, such as a laser, and a light sensing element 104, such as a photo diode. Light 106 is emitted from the light emitting element 102 and passes through a transparent window 108 in a wall 110 of a chamber. When a wafer 112 is not present, the light 106 passes through the transparent window 108 and is detected by the light sensing element 104. Conversely, when a wafer 112 is present, the light 106 passes through the transparent window 108 and is reflected off the surface of the wafer 112, thereby not being detected by the light sensing element 104.
Such optical detection devices, however, have a number of drawbacks. Not only do these devices require the installation of transparent windows, but they also require the use of expensive lasers and photo diodes. Also, the need for a transparent window hampers future retrofits and/or upgrades to the system, as the position of existing windows usually dictates the location of future sensors.
Further, misalignment of any of the optical components in the optical detection device may lead to a detection failure. Still further, reflected or stray light detected by the optical detection device may result in false readings. What is more, maintaining alignment requires continual maintenance and testing, which is both time consuming and costly.
In light of the above, there is a need for an improved wafer position or proximity sensor that addresses the abovementioned drawbacks.
Currently, in semiconductor fabrication systems, optical or proximity sensors are used to sense the position or proximity of a wafer handling robot within the system. The present invention provides a less expensive and more flexible sensing alternative that does not require a transparent viewport window. This provides more flexibility for upgrades, retrofits, new product development, etc.
According to the invention there is provided a proximity sensor. The proximity sensor includes a magnetic field source (first object) configured to generate a magnetic field, a switch plate (second object) made from a ferrous material, and a magnetic field sensor (detector). The magnetic field source and the switch plate are moveable relative to each another. The magnetic field sensor is disposed close enough to the magnetic field source to detect the magnetic field. In use, when the magnetic field source and the switch plate come into proximity of each another, the magnetic field flows from the magnetic field source to the switch plate, thereby disabling detection of the magnetic field and signaling the proximity.
Further, according to the invention there is provided a proximity sensor for a semiconductor wafer fabrication system. The proximity sensor has a chamber including at least one wall having at least a portion thereof made of a non-ferrous material. The proximity sensor also includes a wafer handling robot configured to operate within the chamber. A magnetic field source is coupled to the wafer handling robot and is configured to generate a magnetic field, which is detected by a magnetic field sensor disposed outside of the chamber adjacent the portion of the chamber wall. A switch plate made from a ferrous material is coupled to the wafer handling robot and is configured to make contact with the magnetic field source to disable detection of the magnetic field by the magnetic field sensor.
Still further, according to the invention there is provided a method for determining the proximity of two objects to each another. A magnetic field source (first object) generates a magnetic field, which is detected by a magnetic field sensor (detector). A switch plate (second object) made from a ferrous material and the magnetic field source are brought into proximity of each another, such that the magnetic field flows from the magnetic field source to the switch plate. A loss of magnetic field is sensed at the detector. Finally, the proximity is signaled based on the loss of magnetic field.
Because the proximity sensor is not optically-based, it does not need to be positioned next to a transparent window in the semiconductor fabrication system. Any part of the system that would ordinarily be expected to require service is located on the outside of the chamber walls, i.e., not in the vacuum, for ease of service and reduction of downtime. The proximity sensor is also less expensive and more reliable.